The present invention relates generally to lead elevators or lifts and, more particularly, to hydraulically operated synchronized dual lift systems.
A dual lift system may comprise a parallel pair of vertically extending standards, posts or columns, each having a vertically disposed hydraulic cylinder for vertically raising and lowering a lead carried by the pair of cylinders. The pair of cylinders are actuated from a single source of hydraulic power and the hydraulic circuit contains a flow divider distributing hydraulic flow to each of the cylinders. Typically, the flow divider comprises a spool valve spanning the outlets to the pair of cylinders, the spool being counter-balanced by opposing springs at its opposite ends. This type of flow divider tends to lose its initial stabilizing setting over time, requires frequent readjustment, and is often unreliable in operation under conditions where the lead is off-center relative to the pair of columns.
It is also known to provide a lead elevator in the form of a liftgate comprising a rigid H-frame having a parallel pair of upstanding columns. The columns contain a corresponding pair of vertically disposed hydraulic cylinders having runners interconnected by a transverse heavy stabilizing bar typically supporting a two-section foldable platform actuated on each side by an actuating linkage system. The resulting H-frame is relatively expensive to ship, with or without having the platforms and linkage systems assembled thereto. Further, the heavy stabilizing bar not only entails the penalty of weight but also prevents folding of the platform sections into a fully recessed condition wherein neither the platform sections nor the linkage system parts protrude beyond a rear plane of the pair of columns. As a consequence, the driver of the vehicle on which such a liftgate is mounted must frequently reposition the vehicle away from a dock surface for added clearance to lower the folded platforms into a dock loading position. Even when the vehicle is properly positioned, the presence of the stabilizing bar typically creates a condition in which there is a large gap between the dock and the interior of the vehicle body requiring use of a dock plate to span the gap.
There are also liftgates comprising a vertical pair of columns each containing a runner assembly mounted for vertical reciprocation within the columns. Each runner is typically engaged with interior tracks or surfaces of the companion column by means of rollers. The contact is typically steel on steel and thus induces high wear on the contacting parts and resulting misalignment of the runner assembly relative to its column.
The foregoing and other disadvantages of the foregoing devices are obviated by the present invention.